The present invention relates to a process for producing chlorine dioxide from chlorate ions, acid and hydrogen peroxide
Chlorine dioxide is used in various applications such as pulp bleaching, fat bleaching, water purification and removal of organic materials from industrial wastes. Since chlorine dioxide is not storage stable, it must be produced on-site.
Chlorine dioxide is usually produced by reacting alkali metal chlorate or chloric acid with a reducing agent in an aqueous reaction medium. Chlorine dioxide may be withdrawn from the reaction medium as a gas, as in the processes described by U.S. Pat. Nos. 5,091,165, 5,091,167 and EP patent 612686. Normally, the chlorine dioxide gas is then absorbed into water to form an aqueous solution thereof.
For production of chlorine dioxide in small-scale units, such as for water purification applications or small bleaching plants, it is favourable not to separate chlorine dioxide gas from the reaction medium but to recover a chlorine dioxide containing solution directly from the reactor, optionally after dilution with water. Such processes are described in U.S. Pat. Nos. 2,833,024, 4,534,852, 5,895,838 and in WO 00/76916, and have in recent years become commercial. However, there is still a need for further improvements. Particularly, it has been found difficult to obtain solutions with sufficiently high concentration of chlorine dioxide as required for some applications, like recycle paper bleaching, bagasse bleaching, or small-scale pulp bleaching. A high concentration of chlorine dioxide also may be useful to any application where minimising the water flow is important.
It is an object of the invention to provide a process enabling direct production of chlorine dioxide in an aqueous solution of high concentrations.
It is another object of the invention to provide a process for direct production of chlorine dioxide in an aqueous solution with high production capacity.
It is still another object of the invention to provide an apparatus for performing the process.
It has surprisingly been found possible to meet these objects by providing a process for continuously producing chlorine dioxide comprising the steps of.
feeding chlorate ions, acid and hydrogen peroxide as aqueous solutions to a reactor:
reducing chlorate ions in the reactor to chlorine dioxide, thereby forming a product stream in the reactor containing chlorine dioxide;
feeding motive water to an eductor comprising a nozzle;
bringing the motive water to flow through the nozzle and causing it to flow further through the eductor in an at least partially, preferably substantially, spiral or helical manner,
transferring the product stream from the reactor to the eductor and mixing it with the motive water and thereby forming a diluted aqueous solution containing chlorine dioxide, and;
withdrawing the diluted aqueous solution containing chlorine dioxide from the eductor.
The chlorate ions can be fed to the reactor as an aqueous solution comprising chloric acid and/or a metal chlorate, preferably alkali metal chlorate. The alkali metal may, for example, be sodium, potassium or mixtures thereof, of which sodium is most preferred. Unless chloric acid is used, another acid must be fed to the reactor, preferably a mineral acid such as sulfuric acid, hydrochloric acid or nitric acid, of which sulfuric acid is most preferred. The molar ratio H2O2 to ClO3xe2x88x92 fed to the reactor is suitably from about 0.2:1 to about 2:1, preferably from about 0.5:1 to about 1.5:1, most preferably from about 0.5:1 to about 1:1. Metal chlorate and chloric acid always contain some chloride as an impurity, but it is fully possible also to feed more chloride to the reactor, such as metal chloride or hydrochloric acid. However, in order to minimize the formation of chlorine it is preferred to keep the amount of chloride ions fed to the reactor low, suitably below about 1 mole %, preferably below about 0.1 mole %, more preferably less than about 0.05 mole %, most preferably less than about 0.02 mole % Clxe2x88x92 of the ClO3xe2x88x92.
In a particularly preferred embodiment alkali metal chlorate and hydrogen peroxide are fed to the reactor in the form of a premixed aqueous solution, for example a composition as described in WO 00/76916, which hereby is incorporated by reference. Such a composition may be an aqueous solution comprising from about 1 to about 6.5 moles/liter, preferably from about 3 to about 6 moles/liter of alkali metal chlorate, from about 1 to about 7 moles/liter, preferably from about 3 to about 5 moles/liter of hydrogen peroxide and at toast one of a protective colloid, a radical scavenger or a phosphoric acid based complexing agent, wherein the pH of the aqueous solution suitably is from about 0.5 to about 4, preferably from about 1 to about 3.5, most preferably from about 1.5 to about 3. Preferably, at least one phosphonic acid based complexing agent is present, preferably in an amount from about 0.1 to about 5 moles/liter, most preferably from about 0.5 to about 3 moles/liter, if a protective colloid is present, its concentration is preferably from about 0.001 to about 0.6 moles/liter, most preferably from about 0.02 to about 0.05 moles/liter. If a radical scavenger is present, its concentration is preferably from about 0.01 to about 1 moles/liter, most preferably from about 0.02 to about 0.2 moles/liter. Particularly preferred compositions comprise at least one phosphonic acid based complexing agent selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, 1-aminoethane1,1-diphosphonic acid, amino (methylenephosphonic acid), ethylene diamine tetra (methylenephosphonic acid), hexamethylene diamine tetra (methylenephosphonic acid), diethylenetriamine penta (methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonic acid), and 1-aminoalkane-1,1-diphosphonic acids such as morphollnomethane diphosphonic acid, N,N-dimethyl aminodimethyl diphosphonic acid, aminomethyl diphosphonic acid, or salts thereof, preferably sodium salts. Useful protective colloids include tin compounds, such as alkali metal stannate, particularly sodium stannate (Na2(Sn(OH)6). Useful radical scavengers include pyridine carboxylic acids, such as 2,6-pyridine dicarboxylic acid. Preferably the amount of chloride ions is below about 0.5 moles/liter, most preferably below about 0.1 mmoles/liter, particularly below about 0.03 mmoles/liter.
In the case that sulfuric acid is used as a feed, it preferably has a concentration from about 70 to about 98 wt %, most preferably from about 75 to about 85 wt % and preferably a temperature from about 0 to about 80xc2x0 C., most preferably from about 20 to about 60xc2x0 C., as it then may be possible to operate the process substantially adiabatically. Preferably from about 2 to about B kg H2SO4, most preferably from about 3 to about 5 kg H2SO4 is fed per kg ClO2 produced. Alternatively, the equivalent amount of another mineral acid may be used.
The net reaction resulting in chlorine dioxide generation can be described by the formula;
2ClO3xe2x88x92+2H++H2O2xe2x86x92ClO2+2H2O+O2 
The exact mechanism is complex and is believed to involve a first reaction between chlorate and chloride (even if not added separately always present in sufficient amount as an impurity in chlorate) to give chlorine dioxide and chlorine, followed by reaction of the chlorine with hydrogen peroxide back to chloride. However, considering the net reaction hydrogen peroxide is normally regarded as a reducing agent reacting with the chlorate ions.
The reduction of chlorate ions to chlorine dioxide results in formation of a product stream in the reactor, normally comprising both liquid and foam, and containing chlorine dioxide, oxygen and, in most cases, some remaining unreacted feed chemicals. Chlorine dioxide and oxygen may be present both as dissolved in the liquid and as gas bubbles. When a metal chlorate and a mineral acid are used as feed chemicals, the product stream will then, besides chlorine dioxide and oxygen, contain a metal salt of the mineral acid and normally also some remaining metal chlorate and mineral acid. It has been found possible to achieve a conversion degree of chlorate ions to chlorine dioxide from about 75% to 100%, preferably from about 80 to 100%, most preferably from about 95 to 100%.
The temperature in the reactor is suitably maintained below the boiling point of the reactants and the product stream at the prevailing pressure, preferably from about 20 to about 80xc2x0 C., most preferably from about 30 to about 80xc2x0 C. The pressure maintained within the reactor is suitably slightly subatmospheric, preferably from about 30 to about 100 kPa absolute, most preferably from about 65 to about 95 kPa absolute,
The reactor may comprise one or several vessels, for example arranged vertically, horizontally or inclined. The reactants may be fed directly to the reactor or via a separate mixing device. Suitably the reactor is a preferably substantially tubular through-flow vessel or pipe, most preferably comprising means for mixing the reacants in a substantially uniform manner. Such means may comprise a disk or the like provided with apertures and arranged inside the reactor, wherein metal chlorate and hydrogen peroxide are fed downstream of the disk, while an acid is fed upstream of the disk and brought to flow through the apertures and then mix with the metal chlorate and the hydrogen peroxide. Such an arrangement has been found to give uniform mixing and stable operation of the process as well as ability to vary the production rate with maintained high chemical efficiency, particularly in reactors arranged substantially vertically with the main flow direction upwards. However, it is also possible to simply feed one of the reactants, e.g. the acid, into the feed line for another reactant or mixture of reactants, e.g. a mixture of metal chlorate and hydrogen peroxide.
The length (in the main flow direction) of the reactor used is preferably from about 50 to about 800 mm, most preferably from about 350 to about 650 mm. It has been found favourable to use a substantially tubular reactor with an inner diameter from about 25 to about 300 mm, preferably from about 70 to about 200 mm. It is particularly favourable to use a substantially tubular reactor having a preferred ratio of the length to the inner diameter from about 12:1 to about 1:1, most preferably from about 8:1 to about 4:1. A suitable average residence time in the reactor is in most cases from about 1 to about 1000 seconds, preferably from about 2 to about 40 seconds,
The eductor creates a suction force bringing the product stream, including any liquid, foam and gas therein, to flow into the eductor and mix with the motive water to form a diluted solution containing chlorine dioxide, The motive water is brought to flow in an at least partially spiral or helical manner by any suitable means, such as twisted vanes, internal rifling, or the like, which may be integral or separate from the nozzle and arranged inside or upstream thereof, The nozzle can be of any suitable type and may comprise one or several holes.
The eductor suitably further comprises, in the flow direction from the nozzle, a suction chamber, into which the product stream is transferred from the reactor, and a venturi section, through which the diluted aqueous solution containing chlorine dioxide in such a case is withdrawn. Also eductors with more than one nozzle may be used.
It has been found that the at least partially spiral or helical flow of the motive water increases the production capacity of chlorine dioxide for a given motive water flow, thus enabling production of a product solution with higher chlorine dioxide concentration than what previously was possible only by separating chlorine dioxide gas from the reaction medium and then absorbing it in water, steps not necessary to conduct in the present invention, Thus, it is possible to produce aqueous solutions containing from about 1 to about 4 g/liter of chlorine dioxide, preferably from about 1.5 to about 3.5 g/liter of chlorine dioxide.
The process of the invention is particularly suitable for production of chlorine dioxide in small-scale, for example from about 0.1 to about 100 kg/hr, preferably from about 0.1 to about 50 kg/hr in one reactor, For many applications, a preferred chlorine dioxide production rate is from about 0.1 to about 25 kg/hr, most preferably from about 0.6 to about 10 kg/hr in one reactor. A typical small-scale production unit normally includes only one reactor, although it is possible to arrange several, for example up to about 15 or more reactors in parallel, for example or a bundle of tubes.
The invention further relates to an apparatus for producing chlorine dioxide according to the above described process. The apparatus comprises a reactor provided with feed lines for chlorate ions, hydrogen peroxide and acid, the reactor being connected to an eductor provided with a nozzle for motive water and means for causing the motive water to flow further through the eductor in an at least partially spiral or helical manner.
Preferred embodiments of the apparatus are apparent from the above description of the process and the following description referring to the drawings. The invention is, however, not limited to the embodiments shown in the drawings and encompasses many other variants within the scope of the claims.